Pixel bistability is a desirable attribute for a liquid crystal display (“LCD”) because this eliminates the need constantly to refresh the display or to employ a silicon memory device behind each pixel, which becomes prohibitively expensive as the number of pixels increases. With bistability, only pixels that need to be changed need addressing, and simple matrix addressing may be employed.
Bistable LCDs are known which employ chiral tilted smectic liquid crystals, for example chiral smectic C materials, which exhibit ferroelectricity. However, there are many problems with ferroelectric LCDs, including a paucity of stable, room-temperature materials, wide-temperature-range materials, and structural defects which result from mechanical stress. An attempt to reduce mechanical stress defects is described by V Vorflusev and S Kumar, Science, vol. 283, 1903, 19 Mar. 1999. In the method called phase-separated composite film (PSCOF) technology, an LC is mixed with a photocurable prepolymer and introduced into a cell, one wall of which is provided with a rubbed polyvinyl alcohol alignment layer. The cell is then illuminated with UV light through the wall which does not have the alignment layer, causing phase separation. Phase separation results in a solidified film of polymer on the cell wall closer to the UV source, and an LC film between the polymer film and the alignment layer. Thin LC films can be made, and bonding of the polymer to the substrate adjacent to the LC at a number of random points is said to give increased rigidity and strength and decreased sensitivity to external mechanical deformations.
Because of the problems associated with ferroelectric smectic materials it is desirable to fabricate bistable LCDs using nematic LCs.
U.S. Pat. No. 4,333,708 discloses a multistable nematic LC device in which switching between stable configurations is by the movement of disclinations in response to electric fields.
In WO 91/11747 and WO 92/00546 it is proposed to provide a bistable surface by careful control of the thickness and evaporation of SiO coatings. A first stable planar orientation of the director could be obtained, and a second stable orientation in which the director is at an azimuthal angle (in the plane of the surface) of 90° to the first orientation in the plane of the surface, and tilted by around 30°.
In “Mechanically Bistable Liquid-Crystal Display Structures”, R N Thurston et al, IEEE Trans. on Elec. Devices, Vol. ED-27, No. 11, November 1980, there are described two bistable nematic LC modes which are called “vertical-horizontal” and “horizontal-horizontal”. In the vertical-horizontal mode, both cell walls are treated to give a roughly 45° tilt which permits the directors to be switched between two states in a plane which is perpendicular to the major surfaces of the device. In the horizontal-horizontal mode, the director is switchable between two angles in a plane parallel to the major surfaces of the device.
A bistable nematic display using monostable surface switching has been proposed by I. Dozov et al, Appl. Phys. Lett. 70 (9), 1179, 3 Mar. 1997. Switching in a thin cell is achieved between a low twist and a high twist state by the application of short electric pulses. Both cell wall surfaces have planar anchorings. Hydrodynamically coupled breaking of both anchorings results in the high twist state, and breaking only one anchoring results in the low twist state.
Polarity-sensitive switching has been found in a nematic LC-polymer mixture: R. Bartolino et al, J. Appl. Phys. Vol. 85 No. 5, 2870, 1 Mar. 1999. An LC/prepolymer mixture is polymerised in a cell so as to produce a gradient of concentrated polymer across the cell. It is thought that an asymmetric space charge distribution causes asymmetric electro-optical behaviour. A strong negative current produces a scattering (bright) state, and a weak positive current permits a transition to a homeotropic (dark) state.
Polymeric materials have also found use in modifying surface properties in LCDs. In U.S. Pat. No. 5,155,610 and U.S. Pat. No. 5,262,882 it has been proposed to provide a surface layer of an anisotropic gel or polymer network containing non-reactive LC material on a substrate. The gel can effect an inclined orientation of LC molecules. The surface layer contains liquid crystal molecules, the orientation of at least a part of the molecules being permanently fixed in the anisotropic gel. The angle of inclination of the molecules of the layer differs maximally from a minimum at the interface with the substrate to a maximum at the interface with the LC material when the layer is sufficiently thick. The layer is applied in a thickness to produce a desired angle of inclination at the interface with the LC material.
WO 99/18474 describes the use of oligomers or short chain polymers as lubricants for reducing anchoring energy at surfaces. The oligomers or short chain polymers are substantially non-crystalline in the presence of the LC, and possess a glass transition temperature below the operating temperature range of the device. O. Ou Ramdane et al, Phys. Rev. Lett. 84, No. 17, 24 Apr. 2000 describe the use of highly-mobile grafted polyisoprene or polystyrene for a similar purpose.
U.S. Pat. No. 5,796,459 describes the use of a bigrating alignment structure which induces bistable surface states with different azimuthal orientations. WO 97/14990 and WO 99/34251 describe the use of a monograting surface with a homeotropic local director, which has two stable states with different tilt angles within the same azimuthal plane. The homeotropic alignment is achieved by creating the monograting in a layer of material which causes spontaneous homeotropic orientation or, more practically, by coating the grating surface with a homeotropic inducing alignment agent such as lecithin. This grating surface is used to form a Zenithal Bistable Device or ZBD. WO 01/40853 describes the use of an alignment layer having a primary modulation and a secondary modulation. The primary modulation is formed from a plurality of small alignment areas each having a profiled surface and a homeotropic surface to provide both bistable pretilt alignments and alignment direction to LC molecules. The secondary modulation is formed by the spacing and/or the surface alignment directions of the small alignment areas.
A bistable nematic device was described EP 1 139 151, wherein one cell wall is provided with an array of upstanding features which have a shape and/or orientation to induce the local director to adopt two different tilt angles in substantially the same azimuthal direction. The arrangement is such that two stable molecular configurations can exist after suitable electrical signals have been applied. The features are typically microscopic posts, used to form a Post-Aligned Bistable Nematic device, or PABN.
Typically the depth of gratings or post alignment features of the ZBD or PABN devices is about 1 μm and the ratio depth/width is about 0.6. Gratings this deep are fairly challenging to replicate by mass manufacturing methods, so it is desirable to achieve bistable switching in a nematic device which makes use of shallower gratings.
Switching voltages of prior art grating or post alignment devices range between about 10-60 V and switching speeds range between 50 μs and 50 ms. These speeds can be optimised by changing the shape of the alignment features, but this may also undesirably alter the optical effect. Devices that are less sensitive to the shape of the alignment features are therefore also desirable. Reliable selection of the final state in a bistable device requires that there is some inherent asymmetry in the device; prior art devices either have a grating on one side only, or a different grating on both sides.
The present invention seeks to provide an improved bistable nematic device.